Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitides (AAV) are small vessel vasculitides that typically follow a chronic course and are associated with serious illness and death.Three clinical conditions are recognized: microscopic polyangiitis (MPA); granulomatosis with polyangiitis (Wegener's, GPA); and eosinophilic granulomatosis with polyangiitis (EPA, formerly Churg Strauss Syndrome). Though these conditions have different clinical features, they can have overlapping immunological characteristics.